Shedding light on the nature of hydrogen-bonding jet-cooled aromatic(,m)-(solvent)(,n) clusters using multiresonant laser spectroscopic methods

A powerful combination of laser spectroscopic probes and high level ab initio calculations (completed in collaboration) have been brought to bear on a series of UV chromophore-containing (B = benzene, 2HP = 2-hydroxypyridine, and 2PYR = 2-pyridone), hydrogen-bonded (H-bonded) solvent clusters (W = water and M = methanol) formed in a supersonic expansion. The ultimate goal of this research is to determine the IR spectral signatures of different H-bonding topologies (e.g. chains, cycles, and 3-dimensional networks). In so doing we seek a better understanding of the cooperative effects present in H-bonded networks and of the interactions between the UV chromophore and the solvent sub-clusters. The hydride stretch (O-H, N-H and C-H) region of the IR is employed as a probe of H-bonding and is particularly sensitive to the number, type, and strengths of H-bonds in which it participates, since it vibrates directly against the H-bond. To this end, resonant ion-dip infrared spectra have been collected for some 30 different clusters in the following series: BW n, BMm, BWnMm, 2HP-Wn, and 2PYR-Wn. In BW8 and B2W8, two cubic isomers with S4 and D2d symmetry, which differ only in the orientations of H-bonds within the cube, have been identified based on differences in their IR spectra. Three BW9 isomers, consisting of W9 moieties that should be viewed as expanded cubic W8 structures, have also been found. In BMm clusters (m = 1– 5), methyl C-H stretch modes have been correlated to the previously identified H-bonding topologies. This new probe of H-bonding is employed to unambiguously identify several BWnMm cluster structures with n + m = 2 and 3. The issue of preferential solvation is taken up in the BW nMm clusters with methanol(water) preferring to π H-bond to benzene in WnMm chain(cyclic) sub-clusters. Finally, small water clusters of 2HP and 2PYR have been explored in this model tautomer system. A comparison of the S0 and S1 fluorescence dip infrared spectra of 2PYR-W1,2 provides insight into the changes in vibrational coupling present upon electronic excitation.